The lot area covered by single-family dwellings and structures accessory thereto shall not exceed forty percent of the lot area; duplexes and accessory structures thereto shall not exceed forty percent of the lot area. No residential lot having a width of less than forty feet, a depth of less than seventy-five feet, nor less than twenty feet of street frontage shall be created and in no case shall a lot be created with less than five thousand square feet.